1. Field of the Invention
The present invention relates to a driving apparatus for a multi-color light-emitting display panel that uses capacitive light-emitting elements such as organic electroluminescence elements and a drive method for the same.
2. Description of the Related Art
In recent years, with the trend of increasing the size of display devices, thinner display devices have been required, and a variety of thin display devices have been brought into practical use. An electroluminescence display comprising a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as one of the thin display devices.
As shown in FIG. 1, the organic electroluminescence element comprises at least one layer of an organic functional layer 102, made up of an electron transport layer, a light-emitting layer, a hole transport layer or the like, and a metallic electrode 103, stacked on a transparent substrate 100 of a glass plate or the like on which a transparent electrode 101 is formed. The organic functional layer 102 emits light by applying a positive voltage to the anode of the transparent electrode 101 and a negative voltage to the cathode of the metallic electrode 103, that is, by applying a current across the transparent electrode and the metallic electrode. The electroluminescence display is practicable by using, as the organic functional layer, an organic compound that can be expected to provide good light emission characteristics.
The organic electroluminescence element (hereinafter simply referred to as an EL element) can be represented electrically by an equivalent circuit as shown in FIG. 2. As can be seen from the drawing, the EL element can be replaced by a capacitance component C and a diode characteristic component E that is connected in parallel to the capacitance component. Therefore, the EL element can be considered a capacitive light-emitting element. In the EL element, when a DC light emission drive voltage is applied across the electrodes, electric charge is stored in the capacitance component C. When a barrier voltage or light emission threshold voltage, which corresponds to the element, is exceeded thereafter, a current starts to flow from the electrode (the anode of the diode component E) to the organic functional layer which serves as a light-emitting layer to allow the EL element to emit light at an intensity in proportion to the current.
As shown in FIG. 3, the characteristic of voltage V−current I−luminosity L of such an EL element is very similar to that of a diode, where the current I is extremely small at voltages not larger than the light emission threshold voltage Vth and suddenly increases at voltages equal to or larger than the light emission threshold voltage Vth. In addition, the current I is generally proportional to the luminosity L. In the EL element, when a drive voltage larger than the light emission threshold voltage Vth is applied to the EL element, the element emits light at luminosity proportional to the current corresponding to the drive voltage. On the other hand, when the drive voltage applied thereto is equal to or smaller than the light emission threshold voltage Vth, no drive current flows and the luminosity of light emission remains zero.
As a method for driving a light-emitting display panel that employs such EL elements, known is a simple matrix drive method. FIG. 4 shows the configuration of an example of a drive apparatus that uses the simple matrix drive method for a multi-color light-emitting display panel. In the light-emitting display panel, n cathode lines (metallic electrodes) B1, . . . , Bn are provided in the horizontal direction and 3m anode lines (transparent electrodes) A1R, A1G, A1B, . . . , AmR, AmG, AmB are provided in the vertical direction. EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n are formed at the respective intersections (a total of n×3m). The EL elements E1R, 1, . . . , EmR, n emit red light; the EL elements E1G, 1, EmG, n emit green light; and EL elements E1B, 1, . . . , EmB, n emit blue light. Three EL elements (for example, E1R, 1, E1G, 1, E1B, 1) of each of three primary colors of red, green, and blue, consecutive in each of the cathode lines, form one pixel. The EL elements E1R, 1, E1G, 1, E1B, 1, . . . , EmR, n, EmG, n, EmB, n are arranged in the shape of lattice with one end thereof (the anode line side of the diode component E in the aforementioned equivalent circuit) connected to the anode lines and the other end thereof (the cathode side of the diode component E in the aforementioned equivalent circuit) connected to the cathode lines, corresponding to the intersections of the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB, which are directed along the vertical direction, and the cathode lines B1, . . . , Bn, which are directed along the horizontal direction. The cathode lines are connected to a cathode line scanning circuit 1, while the anode lines are connected to an anode line drive circuit 2 and an anode line reset circuit 3.
The cathode line scanning circuit 1 has scanning switches 51, . . . , 5n, corresponding to the cathode lines B1, . . . , Bn, for determining individually the electric potential of each of the cathode lines, each relaying and supplying either one of a positive potential Vcc which serves as a reverse bias voltage or the ground potential (0V) to a corresponding cathode line.
The anode line drive circuit 2 has current sources 21R, 21G, 21B, . . . , 2mR, 2mG, 2mB, (for example, constant current sources), corresponding to the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB, for supplying drive currents to individual EL elements through the respective anode lines, and drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB. The anode line drive circuit 2 performs on/off control to allow the drive switches to supply currents to individual anode lines. Voltage sources such as constant voltage sources can be used as the drive sources. However, current sources (current circuits that are controlled to provide a desired amount of supply current) are generally used due to a fact that the aforementioned current—luminosity characteristic is stable against a variation in temperature, whereas the voltage—luminosity characteristic is unstable against a variation in temperature. The amount of supply current of the current sources 21R, 21G, 21B, . . . , 2mR, 2mG, 2mB is an amount of current required for the EL elements to sustain a state of emitting light at desired instantaneous luminosity (hereinafter, this state is referred to as the steady light emitting state). In addition, the aforementioned capacitance component C of the EL element is charged with electric charge corresponding to the amount of supply current when the EL element is under a light-emitting state. Accordingly, the voltage across the EL element becomes a specified value Ve (hereinafter, this is referred to as the light emission regulating voltage).
The anode line reset circuit 3 has shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB, which are provided for each of the anode lines. The shunt switches are selected to set the cathode lines to the ground potential.
The cathode line scanning circuit 1, the anode line drive circuit 2, and the anode line reset circuit 3 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1, the anode line drive circuit 2, and the anode line reset circuit 3 to display, in accordance with image data supplied from an image data generation system (not shown), the image to be served by the image data. The light emission control circuit 4 generates a scanning line select control signal for the cathode line scanning circuit 1 to select one cathode line from the cathode lines B1, . . . , Bn, corresponding to a horizontal scanning period of the image data, so that the selected cathode line is set to the ground potential and the remaining cathode lines are supplied with the positive potential Vcc. The positive potential Vcc is applied to EL elements by constant voltage sources connected to the cathode lines to prevent the EL elements, which are connected to the intersections of the driven anode lines and the cathode lines which are not selected for scanning, from producing cross-talk light emission. The positive potential is set such that Vcc=Ve. As the scanning switches 51, . . . , 5n are sequentially switched to the ground potential in each horizontal scanning period, a cathode line set at the ground potential functions as a scanning line which enables the EL elements connected thereto to emit light.
The anode line drive circuit 2 performs light emission control for the scanning line. The light emission control circuit 4 generates a drive control signal (a drive pulse) that shows which EL elements connected to the scanning line is allowed to emit light, the timing, and the duration of time for the light emission, in accordance with pixel color information shown by image data, and supplies the signal to the anode line drive circuit 2. In accordance with the drive control signal, the anode line drive circuit 2 turns on some of drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB to supply drive currents to the corresponding EL elements through the anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB. This allows the EL elements to which drive currents are supplied to emit light in accordance with the pixel color information. Any color can be obtained depending on the light emission luminosity of each of the EL elements in a pixel or depending on the duration of time for light emission within a light emission period.
The reset operation of the anode line reset circuit 3 is carried out in accordance with the reset signal from the light emission control circuit 4. The anode line reset circuit 3 turns on some of the shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB, corresponding to the anode lines, shown by the reset control signal, to be reset, and turns off the remaining shunt switches.
Japanese Patent Laid-Open Publication No.Hei 9-232074, applied by the same applicant as the present applicant, discloses a drive method for performing a reset operation (hereinafter referred to as the reset drive method) in which electric charge stored in each EL element, disposed in a lattice shape, immediately before a scanning line is changed in a simple matrix display panel. This reset drive method is to accelerate the rise time of light emission in EL elements when a scanning line is changed. The reset drive method for a simple matrix display panel will be explained with reference to FIGS. 4 to 6.
The operation that is shown in FIGS. 4 to 6 and described below includes an example in which the cathode line B1 is scanned to allow the EL elements E1R, 1, E1G, 1, E1B, 1 to emit light and thereafter, the scan is transferred to the cathode line B2 to emit the EL elements E2R, 2, E2G, 2, E2B, 2. In addition, for the sake of clarity in explanation, EL elements that are emitting light are indicated by diode symbols, whereas those that are not emitting light are indicated by capacitor symbols. Moreover, the positive potential Vcc applied to the cathode lines B1, . . . , Bn is made equal to the light emission regulating voltage Ve of an EL element.
Referring to FIG. 4, first, only scanning switch 51 is switched over to the ground potential of 0 (V) and the cathode line B1 is scanned. The positive potential Vcc is applied to the remaining cathode lines B2, . . . , Bn by means of the scanning switches 52, . . . , 5n. At the same time, the current sources 21R, 21G, 21B are connected to the anode lines A1R, A1G, A1B by means of the drive switches 61R, 61G, 61B. In addition, the remaining anode lines A2R, A2G, A2B, . . . , AmR, AmG, AmB are switched over to the ground potential of 0 (v) by means of the shunt switches 72R, 72G, 72B, . . . , 7mR, 7mG, 7mB. Thus, in the case of FIG. 4, a voltage is applied to only the EL elements E1R, 1, E1G, 1, E1B, 1 in the forward direction, where drive currents flow in from the current sources 21R, 21G, 21B, as indicated by arrows, to allow only the EL elements E1R, 1, E1G, 1, E1B, 1, to emit light. In this state, the EL elements E2R, 2, E2G, 2, E2B, 2, . . . , EmR, n, EmG, n, EmB , n, which emit no light and are indicated by hatching, are charged in the direction of the polarity shown in the figure.
The following reset control is carried out immediately before the scan is transferred from the light-emitting state of FIG. 4 to the state where the subsequent EL elements E2R, 2, E2G, 2, E2B, 2 emit light. That is, as shown in FIG. 5, all the drive switches 61R, 61G, 61B, . . . , 6mR, 6mG, 6mB are opened, and all the scanning switches 51, . . . , 5n and all the shunt switches 71R, 71G, 71B, . . . , 7mR, 7mG, 7mB are switched over to the ground potential of 0 (V). The anode lines A1R, A1G, A1B, . . . , AmR, AmG, AmB and the cathode lines B1, . . . , Bn are set to the ground potential of 0 (V), thus all being reset. By resetting all, all the anode lines and the cathode lines are made equal to the same potential of 0 (V), so that electric charge stored in each of the EL elements is discharged and thus the charged electric charge in all EL elements disappear instantly.
After the charged electric charge in all of the EL elements is zero, only the scanning switch 52 corresponding to the cathode line B2 is then switched over to the 0 (V) side to carry out scanning over the cathode line B2 as shown in FIG. 6. At the same time, the drive switches 62R, 62G, 62B are closed to connect the current sources 22R, 22G, 22B to the corresponding anode lines A2R, A2G, A2B; and the shunt switches 71R, 71G, 71B, 73R, 73G, 73B, . . . , 7mR, 7mG, 7mB are turned on to give 0 (V) to the anode lines A1R, A1G, A1B, A3R, A3G, A3B, AmR, AmG, AmB. Accordingly, in the case of FIG. 6, a voltage is applied to only the EL elements E2R, 2, E2G, 2, E2B, 2 in the forward direction, where drive currents flow in from the current sources 22R, 22G, 22B to allow only the EL elements E2R, 2, E2G, 2, E2B, 2 to emit light.
The light emission control of the aforementioned reset drive method is to repeat the scan mode or a period for making any one of the cathode lines B1, , Bn active and the subsequent reset mode. The scan mode and the reset mode are carried out for every one horizontal scanning period (1H) of image data. Suppose that the state of FIG. 4 is directly transferred to that of FIG. 6 without carrying out the reset control. For example, the drive currents supplied from the current sources 22R, 22G, 22B, . . . , 2mR, 2mG, 2mB not only flow into the EL elements E2R, 2, E2G, 2, E2B, 2 but also are dissipated to cancel out the electric charge stored in the reverse direction (shown in FIG. 4) in the EL elements E2R, 3, . . . , E2R, n, E2G, 3, . . . , E2G, n, E2B, 3, . . . , E2B, n. Consequently, it will take time to make the EL elements E2R, 2, E2G, 2, E2B, 2 emit light (to make the voltage across the EL elements E2R, 2, E2G, 2, E2B, 2 equal to the light emission regulating voltage Ve).
However, carrying out the aforementioned reset control allows the potential of the anode lines A2R, A2G, A2B to become generally Vcc at the instant of changing the scan to the cathode line B2. Then, charge currents flow into the EL elements E2R, 2, E2G 2, E2B, 2, which should be subsequently allowed to emit light, not only from the current sources 22R, 22G, 22B but also through a plurality of routes from the constant voltage sources connected to the cathode lines B1, B3, . . . , Bn as shown in FIG. 6. These charge currents charge parasitic capacitances (the aforementioned capacitive components C) to allow the EL elements E2R, 2, E2G, 2, E2B, 2 to reach the light emission regulating voltage Ve and to be transferred to the state of light emission. After that, since within a scanning period of the cathode line B2, as described above, the amount of current supplied from each of the current sources is restricted to an amount of current just enough for each of the EL elements to sustain the state of light emission at a light emission regulating voltage Ve, the currents supplied from the respective current sources 22R, 22G, 22B flow into only the EL elements E2R, 2, E2G, 2, E2B, 2, all being dissipated for light emission, and the state of light emission shown in FIG. 6 is sustained.
As described above, according to the reset drive method, all the cathode lines and anode lines are once connected to either the ground potential of 0 (V) or the same potential of positive potential Vcc to reset the EL elements before the process proceeds to the light emission control of a subsequent scanning line. Accordingly, when the scan is changed over to the subsequent scanning line, it is possible to charge the EL elements quickly up to the light emission regulating voltage Ve and thus to provide EL elements which should emit light on the changed scanning line with a quick increase of light emission.
However, EL elements for red, green, and blue colors have element structures and materials, different from each other, so that the EL elements have the characteristics of voltage V−luminosity L, which are different from each other. Therefore, when all the EL elements constituting one pixel emit light to display white color, voltages applied to the both ends of each of the EL elements are different from each other. Thus, it is generally true that each of EL elements for red, green, and blue colors has a different light emission regulating voltage Ve. Therefore, when the reverse bias voltage Vcc is applied to each of the EL elements for red, green, and blue colors by the reset control as described above, and the cathode line for a subsequent scan is selected after the reset control, a difference in time will be produced until voltages across the EL elements, which should be allowed to emit light, on the cathode line selected reaches the light emission regulating voltage Ve of each of the red, green, and blue colors. Thus, since light emission at the light emission regulating voltage Ve did not take place at the same time, a problem of producing a difference in color was present.